Revert "xen/arm: introduce XENFEAT_grant_map_identity"

Revert "xen/arm: introduce XENFEAT_grant_map_identity"

Revert commit id 8d09ef6906ca0a9957e21334ad2c3eed626abe63.
Just keep the definition of XENFEAT_grant_map_identity.

XENFEAT_grant_map_identity is superseeded by GNTTABOP_cache_flush.  As
XENFEAT_grant_map_identity causes additional tlb flushes, it is best to
remove the feature entirely now.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>